Our recent observations on platelet actomyosin indicate the cooperative interaction of the substrate (ATP) molecules and point to the regulatory nature of this protein. The allosteric function of platelet actomyosin will be characterized as follows: (1) Studies will be carried out to obtain direct evidence regarding the total number of ATP binding sites using equilibrium and spectroscopic techniques. (2) The feedback effects of the products of reaction will be studied. (3) The sensitivity of platelet actomyosin towards heat, pH mercurials urea and various mono and divalent cations will be undertaken. (4) Role of Ca 2 ion in regulating the contractile process in platelets will be investigated. (5) Studies will be made to localize the active and the regulatory site. The possible role of subunits of platelet actomyosin in this regard will be investigated. (6) Because of the similarities of platelet contractile proteins to other smooth muscle proteins, similar studies will also be made with gizzard, arterial and uterus contractile proteins. (7) From these studies a possible mechanism to explain contractility in platelets and other smooth muscle systems will be established.